Astronomers Report The First Detection Of A Noble Gas In A Comet

Southwest Research Institute (SwRI) News

For release after 10:00 a.m. EDT, June 5, 2000

Astronomers report the first detection of a noble gas in a comet and new
clues to the origin of Hale-Bopp

San Antonio, June 5, 2000 -- Astronomers announced today at the American
Astronomical Society meeting in Rochester, New York, that they have
detected argon, a scientifically valuable noble gas, in comet Hale-Bopp.
This discovery constitutes the first-ever detection of a noble gas in a
comet. Noble gases provide valuable tracers of the thermal history and,
therefore, clues to the origins of comets.

The discovery was made by a team of four astronomers from Southwest
Research Institute=AA (SwRI), based in San Antonio, Texas, collaborating
with three colleagues from the University of Colorado, the University of
Maryland, and the Observatoire de Midi-Pyrenees in France. The study was
supported by NASA.

The data on comet Hale-Bopp were obtained in the form of ultraviolet
spectra during a NASA high-altitude suborbital research rocket flight on
the evening of March 29, 1997, just as comet Hale-Bopp made its closest
approach to the sun. According to team leader and Principal Investigator
Dr. Alan Stern, director of the SwRI Space Studies Department, "The argon
signals are weak, but unmistakable. We had previously suspected their
presence, but were able to recently confirm the result when we
cross-compared two independent spectra obtained by our rocket
instrument back in 1997."

Adds co-investigator Dr. David Slater, a senior research scientist at SwRI,
"Hale-Bopp was among the brightest comets ever witnessed, and surely
the brightest comet in modern times. The detection of argon would not
have been possible except for Hale-Bopp's unusually high brightness."

Because noble gases do not interact chemically with other elements and
because noble gases are easily lost from icy bodies like comets at very
low temperatures through processes much like evaporation, their presence
or absence provides a way of measuring the thermal history of comets.
University of Maryland astronomer and team member Dr. Michael A'Hearn
explains, "That's the reason cometary astronomers have wanted to detect
noble gases for so long. The advance of technology combined with the
brightness of Hale-Bopp made this goal a reality."

Interestingly, the team's spectra showed that the argon abundance in
Hale-Bopp was so high that it indicates the comet has always been quite
cold and likely formed in the deep outer reaches of the solar system, far
beyond its once-suspected birthplace in the somewhat warmer Jupiter
zone. "Our results indicate that Hale-Bopp was likely formed in the
Uranus-Neptune zone," says Stern. The high argon abundance of Hale-Bopp
may also help explain the unexpected findings by the Galileo Jupiter entry
probe, which found that Jupiter has an argon abundance similar to comet
Hale-Bopp. "Perhaps Jupiter was seeded with extra argon by the impact
of many comets like Hale-Bopp early in the history of the solar system,"
remarks Stern.

The detection of argon in Hale-Bopp has whet the scientists' appetite for
more noble gas data on comets. The team is preparing an instrument called
the ALICE Ultraviolet Spectrometer for NASA to fly to comet Wirtanen
aboard the European-U.S. Rosetta comet orbiter mission to be launched in
2003. The team has proposed a series of additional NASA rocket launches
in 2002 and 2003 to search for argon and other noble gases, even before
the Wirtanen orbiter mission is launched. Stern says, "Using this even
more sensitive generation of instruments, we look forward to comparing
different comets to one another to learn about the diversity of cometary
birthplaces."